Base current-control circuit of an output transistor

Abstract: A base current-control circuit of an output transistor (Q out ) comprises a detector (Q s ) for detecting a load current of the output transistor; a current-voltage converter (2) for converting a detected current to equivalent voltage; and a base current generator (7) for generating a base current in accordance with ON/OFF signals of a switching transistor (Q sw )to drive the output transistor in the use of detecting voltage and reference voltage.

Description: The present invention relates to a base current-control circuit of an output transistor. More particularly, this invention relates to a base current-control circuit of an output transistor which changes the base current of the output transistor in accordance with the load current of the output transistor for maximizing efficiency in the use of electric power. Electronic equipment often includes an output transistor to drive an external device. The output transistor is designed to carry a large current and supplies a load with a current of a collector which is controlled by a base current. Figure 1 shows an output terminal of electronic equipment comprising an output transistor Q out and a load R L . Vcc is a source of electric power. When an input signal processed by the electronic equipment triggers a switching transistor Q SW , the switching transistor is turned on or off. When the switching transistor Q SW is turned on, the output transistor is turned on. When the switching transistor Q SW is turned off, the output transistor is turned off. In detail, when the switching transistor is turned on, a diode D 1 connecting a transitor base with the collector is also turned on, and a constant-voltage source 4 loads a resistance R b with V ref voltage. The voltage at node A, V A is the same as the total of V ref and a diode voltage V D1 and the voltage at node B, V B is equal to the subtraction of the voltage between a base and an emitter of transistor Q 1 from node A voltage V A . V B is the same as V ref + V D1 - V BE , Q 1 and if V D1 is the same voltage as the V BE , Q1 V B can be V ref . The collector current of transistor Q 1 , namely a base current I B of the output transistor Q out is the same as V B /R b which is V ref /R b , and I B is constant. I B is decided by the resistance R b and a constant voltage and is independent of the magnitude of the load R L of the output transistor Q out . So, regardless of load current I o an invariable base current I B flows and electric power is dissipated unnecessarily. If the base current I B is controlled in accordance with the magnitude of the load current I o , then electric power would be used efficiently. EP-A-514980 discloses a driving circuit for a switching transistor comprising a detector for detecting a current dependent on the load current of the transistor and means to generate a base current to drive the transistor. EP-A-384513 discloses a circuit for regulating the base current of a semiconductor power device which acts to maintain constant the ratio between the emitter current and base current of the device. The present invention is directed to a base current-control circuit of an output transistor for maximizing efficiency in the use of electric power. This base current-control circuit of the output transistor controls the base current in accordance with the load current of the output transistor. According to the present invention there is provided a base current-control circuit of an output transistor comprising: a detector for detecting a load current of said output transistor, a base current generator for generating a base current to drive the output transistor, and characterised by a current-voltage converter for converting the detected current to an equivalent voltage, wherein the base current generator generates base current in accordance with ON/OFF signals of a switching transistor to drive the output transistor, by the use of the detected voltage and a reference voltage. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram illustrating an output terminal of previously proposed electronic equipment; Figure 2 is a block diagram illustrating embodiments of the present invention; Figure 3 shows an embodiment of the present invention; and Figure 4 is a graph comparing operation characteristics between the prior art and the present invention. Base current I B of an output transistor is shown as a simple linear function of a load current I O . So the load current, an independent variable, decides to the base current, a dependent variable. The base current is controlled by the load current. Referring to Figure 2, the load current of a driving terminal 8 connected to the output transistor is a detected current I sense detected by a load current detector. A current-voltage converter converts the detected current to equivalent voltage V sense . An output V ref from a constant-voltage source 4 and detected voltage V sense are input to a base current-control voltage generator, which outputs a base current-control voltage. The base current-control voltage is input to a switch. The signal from an output transistor ON/OFF controller is input to the switch and the base current-control voltage, via the switch, flows into a base current generator 7. The controlled base current I B from the base current generator 7 is input to the output transistor of a driving terminal 8. The base current I B is controlled by the load current. Figure 3 shows one embodiment of the present invention. A transistor Q S and an output transistor Q out are set up in parallel to detect the load current from the driving terminal 8. The output transistor Q out is a PNP type transistor. The transistor Q S for detecting the load current is also a PNP type. A detecting current I sense is decided by the rate of an emitter area between the transistor Q S and the output transistor Q out . When the emitter area of Q S /the emitter area of Q out is K, I sense is K x I 0 . As K is fixed, I sense changes in proportion to I o . V be,QS which is the voltage between the base and the emitter of the transistor Q S is the same as V be , Q out which is the voltage between the base and the emitter of the output transistor Q out . This is an equivalent formula V be ,Q S = V be ,Q out V T ln I c ,Q s I s x K = V T ln I c , Q out I s where V T is the transistor thermal voltage, I s is a saturation current and K is the emitter area of Q S /the emitter area of Q out . Therefore, I c ,Q s , a collector current of Q s is K x I c ,Q out . K is in the range from 1/100 to 1/1000. Current-voltage converter 2 converts detected load current I sense to an equivalent voltage. In an embodiment, resistance R s converts because the detected load current I sense flows into the resistance R s and then a voltage drop arises. The size of voltage is in proportion to the size of an inflow current. The detected voltage V sense is I sense x R s . Referring to Figure 2, a base current-control voltage generator 3 receiving the detected voltage V sense and reference voltage V ref outputs a base current-control voltage, which is applied to node C. Reference voltage V ref in series with resistance R s added to the voltage on resistance R s makes voltage on node C. At this point, reference voltage V ref is base current-control voltage of the output transistor in the absence of a load. As shown in the circuit, V ref is fixed, so base current-control voltage V c changes in proportion to I sense and outputs to node C. This is shown as V ref + K x I o x R s and it is a simple linear function of I o . Referring to Figure 2, base current-control voltage V c inputs to switch 6. The input signal is an output signal of the output transistor ON/OFF controller in internal electronic equipment. The switching transistor Q sw turns ON or OFF in accordance with these signals. When the switching transistor turns on, base current-control voltage V c flows into the transistor Q 1 , a kind of buffer, and base current-control voltage appears on resistance R b connected to the emitter of NPN type transistor Q 1 . This current shows as V c /R b . This is the base current I B . The formula 1 is as follows. IB = V ref + K x I o x R s R b = V ref R b + K x R s R b x 1 0 A base current generator 7 of Figure 2 can be embodied in the transistor Q 1 as shown in Figure 3. A collector current of the transistor Q 1 , that is, the base current I B of the output transistor is controlled by I o in the manner shown by formula 1. The voltage on node B is the sum of V ref and K x I o x R s . Figure 4 is a graph showing the operation characteristics compared with the prior art. The vertical and horizontal axes show respectively the base current I B and the load current I o . In the prior art shown as line A, the base current I B is invariable regardless of the load current I o . However, in the present invention (as per formula 1), the graph B indicates the base current I B . The output current is related to the load, which receives driving power from the suitable amount of base current I B . If the base current in the prior art and the present invention are I B1 and I B2 respectively at the same level of power voltage V cc and the load current I o , losses are reduced by as much as(I B1 - I B2 ) x V cc , which is an amount of current of power.

Claims: A base current-control circuit of an output transistor (Q out ) comprising: a detector (Q s ) for detecting a load current of said output transistor, a base current generator (7) for generating a base current to drive the output transistor, and characterised by a current-voltage converter (2) for converting the detected current to an equivalent voltage, wherein the base current generator generates base current in accordance with ON/OFF signals of a switching transistor (Q sw ) to drive the output transistor, by the use of the detected voltage and a reference voltage (V ref ). A circuit as claimed in Claim 1, wherein said load current detector (Q s ) comprises the same conductive type transistor as the output transistor (Q out ) to drive said output transistor symmetrically in parallel. A circuit as claimed in Claim 1 or Claim 2, wherein said current-voltage converter receiving the detected current comprises a resistor (R s ) connected in series with a reference voltage (V ref ). A circuit as claimed in any one of the preceding claims, wherein the base current is the linear sum of the reference voltage and the detecting voltage corresponding to the load current, and the base current is applied to said base current generator which comprises a transistor (Q 1 ) and a resistor (R 6 ) connected to its emitter. A circuit as claimed in any one of the preceding claims, wherein the current on said emitter resistor is the base current of said output transistor and is a simple linear function of the load current (I o ). A circuit as claimed in any one of the preceding claims, wherein the detecting current is the multiplication of the emitter of the transistor (Q s ) detecting the load current, the ratio of the emitter area in the output transistor and I o . A circuit as claimed in any preceding claim and further comprising a control signal generator (3) for generating a base current-control voltage by the use of the detected voltage and reference voltage. A circuit as claimed in Claim 7, wherein a switching means (Q sw ), outputting the signals to the driving terminal, is formed between the base current generator and the base current-control voltage generator.

Inventors: IM CHANGSIK, IM, CHANGSIK
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO., LTD.
Application year: 1994
Main classification: G05F3
IPC codes: G05F1, H03K17, G05F3